Procedure and control unit for heating up a catalyst arranged in the exhaust gas system of a supercharged combustion engine

ABSTRACT

Suggested is a procedure for heating up a catalyst in the exhaust gas system of a charged combustion engine by adding air to the exhaust gas system, whereby the added air is taken from a suction system of the combustion engine in the direction of the air current behind a compressor of an exhaust gas turbo charger that is arranged in the suction system. The procedure distinguishes itself thereby that the combustion engine is driven with a direct injection of fuel in its combustion chambers, whereby it is driven after a start-up with an apportionment of a fuel amount, which has to be injected before the beginning of a combustion, into at least two partial injections per ignition, and with a suboptimal ignition angle efficiency. A second independent claim concerns a control unit, which is customized for controlling the course of the procedure.

TECHNICAL FIELD

The present invention concerns a procedure for heating up a catalyst inthe exhaust gas system of a charged combustion engine by adding air tothe exhaust gas system, whereby the added air is taken from a suctionsystem of the combustion engine in the direction of the air currentbehind a compressor of an exhaust gas turbo charger that is arranged inthe suction system. The invention furthermore concerns a control unit ofthe combustion engine that is customized for controlling the course ofthe procedure.

BACKGROUND

Such a procedure and such a control unit are both known from DE 100 62377 A1. The heating up of a catalyst by injecting secondary air into arich exhaust gas atmosphere is also already known from the publicationof DE 100 62 377 A1. The secondary air is usually injected behind theoutlet valves of the combustion engine and promptly reactsexothermically with a rich exhaust gas atmosphere, which results fromcombustions of rich (air lambda value lower than 1) combustion chamberfillings of the combustion engine. A separate secondary air pump that iselectrically driven is usually used for injecting the secondary air.

The procedure that works with secondary air injection can be associatedwith a group of heating procedures, which have in common that theheating-up takes place by the reaction heat of chemical reactions thattake place in the exhaust gas. Furthermore interventions into thecombustion engine control for heating-up the catalyst are known, whichcause an increase of the exhaust gas temperature and/or the exhaust gasmass flow.

It is known for example to produce an extremely high amount of heat inthe exhaust gas in an after-start phase of the combustion engine,without changing the engine power that has been raised during idling ofthe combustion engine nor the idle speed of about 1.200 min-1 that hasbeen raised in the after-start phase. This is achieved at a combustionengine with direct fuel injection by injecting a first amount of fuel inthe suction stroke and a second amount of fuel in the compressionstroke. This causes a layered fuel apportionment in the combustionchamber with a zone, which results from the injection of the secondamount with a comparably rich and therefore well ingnitable fuel/airmixture around the ignition plug. This operation of the combustionengine is also called homogeneous split mode, whereby ‘split’ refers tothe apportionment of the injections.

The above mentioned DE 100 62 377 A1 is based on a two-stage concept forsupercharging. The two-stage concept thereby provides an exhaust gasturbo charger in one embodiment, whose shaft is driven by anelectromotor. By this drive (1. stage) the so-called turbo ‘lag’ shallbe minimized at operating point changes. As it is generally known theturbo lag develops, because the turbine initially has to be acceleratedduring a sudden torque demand from an operating point with a low exhaustgas mass flow, in order to establish the necessary boost pressure on thecompressor side. The resulting delay is reduced by the supportingelectric drive. The second stage is equivalent to the traditional driveof the turbine by a sufficient big exhaust gas enthalpy.

This two-stage supercharging concept, which has nothing to do with acatalyst heating process, is used in DE 100 62 377 A1 in order toreplace the separate secondary air pump. Therefore the turbo charger iselectrically driven when the catalyst has to be heated. Thereby italready produces a certain boost pressure also in operating points witha low exhaust gas enthalpy, which is sufficient in order to let air flowout of the suction system over a pipe connection past the combustionchambers of the combustion engine into the exhaust gas system. Thereby aseparate secondary air pump can be waived at two-stage superchargingconcepts with turbo chargers that are supported by an electrical drive.But the injecting of the secondary air requires an electric drive evenat such two-stage supercharging concepts.

SUMMARY

With this background the task of the invention is to provide a procedureand a control unit, which allow a heating of catalyst in the exhaust gasof a combustion engine that is charged with a exhaust gas turbo charger,which uses secondary air without a separate secondary air pump andwithout an electrical drive of the turbo charger or a compressor that isarranged in the suction pipe.

This task is solved with the features of the independent claims.

The operation of the combustion engine with a direct injection of fuelin its combustion chambers and with an apportionment of a fuel amountthat has to be injected before the beginning of a combustion, into atleast two partial injections per ignition and combustion chamber, whichtakes place after a start-up, provides very stabile combustions, whichallow the very late ignition angle. Late ignition angles up to 25degrees after top dead center can be adjusted at air- and wall-formedcombustion procedures, and at jet-formed combustion procedures evenlater ignition angles between 25 up to ca. 40 degrees after top deadcenter can be adjusted at a stabile engine speed behavior and atcontrollable raw emissions during idle. Thereby the ignition angleefficiency, which can be understood as the quotient between the torqueat a delayed ignition angle in the numerator and the torque at anoptimal ignition angle for a maximum torque development, sinks.

The efficiency loss causes a higher exhaust gas temperature andtherefore a higher exhaust gas enthalpy due to thermodynamic regularity.Furthermore the combustion engine has to be operated with highercombustion chamber fillings at a delayed ignition, in order tocompensate the torque loss that goes along with the efficiency failure.At the given ignition angles increases of the combustion chamberfillings occur up to values of over 75% of the maximum volume that ispossible at normal conditions. This causes an increased exhaust gas massflow, which also increases the exhaust gas enthalpy. With an increasingexhaust gas enthalpy the driver input that is transferred on to theturbine of the exhaust gas turbo charger increases. Altogether thisresults in a comparably high exhaust gas amount, whose temperature iscomparably high due to the bad ignition angle degree, so that a maximumheat flow (enthalpy flow) adjusts in the exhaust system.

The achieved increase of the exhaust gas enthalpy causes alreadyconsidered on its own a fast heating of the exhaust system. Furthermorethe increase causes without a supporting electrical drive within a fewseconds after a cold start that the turbo charger establishes a boostpressure and therefore a pressure drop or a scavenging loss to theexhaust gas, which is also sufficient big enough at low engine speeds inorder to let air stream out of the exhaust gas system over a pipeconnection past the combustion chambers of the combustion engine intothe exhaust gas. Thereby a separate secondary air pump can be waivedeven at one-stage supercharging concepts, which work withoutelectrically supporting turbo chargers and without an additionalcompressor (for example roots-injector, compressor) that is electricallyor mechanically driven by the combustion engine. The invention thereforetakes advantage of the already known homogeneous split mode at asupercharged combustion engine for a boost pressure increase, in orderto achieve a scavenging loss (pressure drop) between the suction systemand the exhaust gas system that is sufficient for a secondary airinjection.

Further advantages accrue from the dependent claims, the description andthe attached figures.

It shall be understood that the previously mentioned and the followingfeatures that still have to be explained cannot only be used in thestated combination, but also in other combinations or alone withoutleaving the scope of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are shown in the drawings and furtherexplained in the following description. It is schematically shown in:

FIG. 1 is a combustion engine with a gasoline direct injection and acontrol unit;

FIG. 2 is an injection model, which is used in the embodiment of theprocedure according to the invention;

FIG. 3 is a flow diagram for illustrating the procedure aspects of theinvention; and

FIG. 4 is timely correlating courses of operating parameters of thecombustion engine during the implementation of an embodiment accordingto the procedure.

DETAILED DESCRIPTION

In particular FIG. 1 shows a combustion engine 10 with at least onecombustion chamber 12, which is sealed up with a piston 14. Fillings ofthe combustion chamber 12 with a mixture of fuel and air are ignited byan ignition plug 16 and afterwards combusted. In a preferred embodimentthe combustion engine 10 is optimized for a jet-formed combustionprocedure. Referred to as a combustion procedure is the way of themixture formation and energy transformation in the combustion chamber12. The jet-formed combustion procedure distinguishes itself therebythat the fuel in immediate proximity to the ignition plug is injectedand evaporates there. This requires an exact positioning of the ignitionplug 16 and fuel injector and a precise jet direction, in order to beable to ignite the mixture at the proper point of time.

An exchange of the filling of the combustion chamber 12 is controlledwith gas change valves 18 and 20, which are opened and closedphase-synchronically with the movement of the piston 14. The differentpossibilities for operating the gas exchange valves 18 and 20 are knownto the technician and are not shown in detail in FIG. 1 due to clarity.When the inlet valve 18 is open and the piston 14 is running downwards,thus in the suction stroke, air flows from a suction system 22 into thecombustion chamber 12. By using an injector 24 fuel is dosed to the airin the combustion chamber 12. Exhaust gas that results from thecombustion of the combustion chamber fillings is ejected into an exhaustgas system 28, which has at least one three-way catalyst 30, when theoutlet valve 20 is opened. Generally the exhaust gas system 28 containsseveral catalysts, for example a pre-catalyst 30 that is build-in nearthe engine and a main catalyst 32 that is build-in far from the engineand that can be a three-way catalyst or a NOx-storage catalyst.

The combustion engine 10 provides a turbo charger 34 with a turbine 36and a compressor 38. The turbine 36 is arranged between a manifold 40and the pre-catalyst 30 in the flow path of the exhaust gases. By usinga waste gate valve 42 the pressure drop over the turbine 36 can belimited. A secondary air duct 44 with a secondary air valve 46 liesbetween the suction system 22 and the exhaust gas system 28. When thesecondary air valve 46 is opened and when there is a sufficient pressuredrop from the suction system 22 (before the inlet valve 18) to theoutlet of the secondary air duct 44 into the exhaust gas system 28, airflows from the suction stroke 22 past the combustion chambers 12 of thecombustion engine 10 into the exhaust gas system 28 before the catalyst.

The combustion engine 10 is controlled by a control unit 48, whichtherefore processes signals of different sensors, which illustrateoperating parameters of the combustion engine 10. These are in theincomplete illustration of FIG. 1 a rotation angle sensor 50, whichdetermines an angle position °KW of a crankshaft of the combustionengine 10 and therefore a position of the piston 14, an air mass sensor52, which determines an air mass mL that flows into the combustionengine 10, a pressure sensor 54, which determines the pressure p in thesuction stroke 22 before the inlet valve 18, and, optional, one orseveral exhaust gas sensors 56, 58, which determine a concentration ofan exhaust gas component and/or a temperature of the exhaust gas.

In the embodiment of FIG. 1 the exhaust gas sensor 56 is a lambdasensor, which determines an oxygen concentration in the exhaust gas as ameasure of an air ratio L (L=lambda), while the sensor 58 determines anexhaust gas temperature T at the inlet of the pre-catalyst 30. The airratio lambda is known to be defined as the quotient of an actuallyavailable air mass in the numerator and an air mass that is required fora stoichiometric combustion of a certain fuel mass in the denominator.Air ratios lambda higher 1 represent therefore an air surplus, while airratios lambda smaller 1 represent a fuel surplus. As long as the exhaustgas system 28 provides an exhaust gas temperature sensor 58, it can bealso arranged in a different position of the exhaust gas system 28, forexample at the inlet of the main catalyst 32. This especially applieswhen the main catalyst 32 is a NOx-storage catalyst.

The control unit 48 creates corrective signals from the signals of thisand if necessary further sensors in order to control actuators forcontrolling the combustion engine 10. In the embodiment of FIG. 1 theseare especially a corrective signal S_L for controlling a throttle valveposition sensor 60, which adjusts the angle position of a throttle valve62 in the suction system 22, a signal S_K, with which the control unit48 controls the injector 24, a corrective signal S_Z, with which thecontrol unit 48 controls the ignition plug 16 or the ignition system 16,which also provides inductors and/or condensers for producing theignition voltage, and a corrective signal S_SLE, with which the controlunit 48 controls the inlet profile of the secondary air valve 46, aswell as a signal S_WG for controlling the waste-gate-valve 42.Analogously to the illustration of the sensors it also applies to thedepicted actuators, that the illustration of FIG. 1 is not complete andthat modern combustion engines 10 can provide further actuators asexhaust gas recirculation valves, tank ventilation valves, actuators forvariable controls of the gas exchange valves 18, 20 etc.

Besides the control unit 48 is customized especially programmed toimplement the suggested procedure and/or one of its embodiments and/orto control a corresponding course of procedure.

In a preferred embodiment the control unit 48 converts performancerequirements of the combustion engine 10 into a nominal value for thetorque that has to be produced altogether by the combustion engine 10,and apportions these torques into torque rates, which are influenced bythe corrective signals S_L for the filling control, S_K for the fuelmetering, S_Z for the ignition control and S_WG for the boost pressurecontrol. The filling rate is adjusted with the corrective signal S_L bya corresponding setting of the throttle valve 62 or a variablecontrolling of inlet valves 18. The fuel rate is adjusted with thecorrective signal S_K basically by the injected fuel mass and the way ofthe apportionment of the fuel mass that has to be injected into one orseveral partial injections as well as the relative status of the partialinjections to each other and to the movement of the piston 14, thus byan injection timing. The maximal torque that is possible at the presentair filling results from optimal air ratio lambda, optimal injectiontiming and optimal ignition angle.

FIG. 2 shows an injection model, which is used at the embodiment of theprocedure according to the invention. Thereby the injector pulse widthsti_1 and ti_2 are each put in as high level over the crankshaft angle°KW of a working cycle from a suction stroke stroke_1, a compressorstroke stroke_2, a working stroke stroke_3 and an outlet strokestroke_4. Upper top dead centers are labeled as OT.

In particular FIG. 2 shows an injection model M_1 for a homogeneoussplit operation for maximized exhaust enthalpy with a first partialinjection ti_1, which takes place in the suction stroke stroke_1 and asecond partial injection ti_2, which takes place later. The secondpartial injection ti_2 takes definitely place before the ignition, whichis caused at the crankshaft angle KW_Z. As already mentioned KW_Z ispossibly very late in the range of 10° to 35° KW after the ignition-ot,so that the second partial injection ti_2 can also be completely orpartially in the working stroke stroke_3. But it is definitely beforethe ignition. Instead of an apportionment into two partial injectionsthe fuel amount that is injected with the first injection model M_1 canalso be apportioned into more than two partial injections. Thepossibility of apportioning is limited by the dosing ability of smallquantities of the injector 24. The apportionment into at least twopartial injections, of which the earlier preferably takes place in thesuction stroke stroke_1 and the latter definitely in the same workingstroke for the ignition, is significant for the model M_1, whereby theair ratio lambda in the combustion chamber (thus without secondary air)is smaller than 1 and an air ratio lambda in the exhaust gas (thus withsecondary air) is higher than 1.

FIG. 3 shows a flow diagram of procedure aspects of the invention. Aftera start-up of the combustion engine 10 in step 64 initially its enginespeed n is determined in step 66 and compared to a threshold value n_SEin step 68. An exceeding of the threshold value n_SE branches theprocedure to step 70, in which the described homogeneous split mode HSPwith retarded ignition and increased filling is activated. In apreferred embodiment the combustion engine 10 is thereby operated almostcompletely de-throttled, whereby an almost complete de-throttling meansan operation with at least 75% of the maximal filling that is possibleunder the same conditions.

Simultaneously or quickly afterwards the secondary air valve 46 isopened in step 72 at a sufficient boost pressure. The opening can forexample take place with a fixed time delay of the order of a few secondstowards the activating of the homogeneous split mode or depending on theexceeding of a boost pressure threshold value. Subsequently in step 74 aparameter A is established and determined, which shows the effect of thesecondary air injection. A time meter reading or a constant thatcharacterizes the temperature of the turbo charger 34, the manifold 40or of a catalyst 30, 32 are preferred as a parameter. Combinations ofsuch constants are also possible. The parameter A is compared to athreshold value S_A as a termination criteria in step 76. When exceedingS_A the homogeneous split mode is terminated in step 78, the secondaryair valve 46 is closed and branched in step 80 in a normal operation ofthe combustion engine 10, in which no special measures for increasingthe exhaust gas enthalpy are activated. The transfer can also take placestep-by-step by closing the secondary air valve 46 first and thenterminating the homogeneous split mode. The order can also be reversed.

The effect of the procedure according to the invention is illustrated bythe time course of the engine speed n, the boost pressure p and acontrol bit SB that are shown in FIG. 4. Before the point of time t=0the combustion engine 10 stands still. Therefore its engine speed n thatis shown in FIG. 4 a initially equals zero and the boost pressure p thatis shown in FIG. 4 b corresponds with the surrounding pressure of about1000 mbar. The value of the control bit SB that is shown in FIG. 4 c isstill low.

A starter accelerates the combustion engine 10 at the point of time t0onto a starter engine speed of a little over 200 min-1. Withconstituting combustions in the combustion chambers 12 the engine speedn of the combustion engine 10 increases more and exceeds a startingengine speed threshold of about 400 min-1 at the point of time t1.Subsequently it quickly levels out at an increased idle engine speed ofabout 1.200 min-1. Due to the suction of the first combustion chamberfillings from the suction system 22 at a turbine 36 that is still notrotating or still not rotating fast the boost pressure p before theinlet valves 18 sinks initially. When exceeding the starting enginespeed threshold at the point of time t1 the after-starting phase begins.The control bit SB from FIG. 4 c is set on its high level. The procedureaccording to the invention or one of its embodiments is implemented at ahigh level.

In order to provide a high enthalpy flow in the exhaust gas during thisafter-starting phase, the control unit 48 provides suboptimal ignitionangles over the corrective variable S_Z, which cause a torque loss overthe therefore reduced ignition angle efficiency, which is compensated byan increased filling of the combustion chambers 12 that is produced bycorrective signals S_L. The turbine 36 of the exhaust gas turbo charger34 is quickly accelerated by the enthalpy flow in the exhaust gas thatis high due to the almost complete de-throttling, so that the boostpressure p increases quickly up to values of over 1200 mbar. During suchboost pressures the pressure difference between the boost pressure onthe fresh air side of the secondary air duct 44 and the exhaust gas sideof the secondary air duct 44 is big enough in order to let fresh airfrom the suction system 22 flow into the exhaust gas system 28 at anopened secondary air valve 46.

Therefore the control unit 48 opens the secondary air valve 46 byreleasing an opening corrective signal S_SLE. By an additional influenceof the fuel corrective signals S_K an air ratio lambda is altogetheradjusted in the exhaust gas in the over-stoichiometric operation, forexample an air ratio lambda=1,1. Depending on the amount of the freshair that has been injected into the exhaust gas, the air ratio lambda inthe combustion chamber 12 is adjusted on to correspondingly lowervalues, which can also lie in the under-stoichiometric operation(lambda<1, fuel surplus). Thereby a good ignition ability and a stabilecombustion of the fuel/air mixture that is comprised in the combustionchambers are achieved. Simultaneously the over-stoichiometric air ratioin the exhaust gas is very important especially in the first phase aftera start finish, because the still cold pre-catalyst 30 can not reducehydrocarbons yet. Therefore the only possibility to limit thehydrocarbon emissions that are stored in the environment is to limit theraw emissions of the combustion engine 10. This limitation is a desiredresult of the operation with an air ratio lambda bigger than 1 in theexhaust gas.

A high exhaust gas amount is produced by the increased filling, whichhas furthermore a comparably high temperature due to the suboptimalignition angle efficiency and which provides a oxygen surplus.Altogether a high heat flow or enthalpy flow is therefore produced. Assoon as a termination criteria is fulfilled at the point of time t2, theincrease of the exhaust gas enthalpy is terminated. The engine speed nof the combustion engine 10 falls then back on its normal idle enginespeed, which lies typically between 500 and 100 min-1. The de-throttlingthat exceeds the necessary scope during normal operation is terminated.Thereby the pressure p between the throttle valve 62 that is than lessopened and the inlet valves 18 drops a lot. In the drawing of FIG. 4 thepressure sinks up to about 400 mbar, whereby the actual value can varyform combustion engine to combustion engine and also depending on otherconditions. The low pressure is then not sufficient for a secondary airinjection, so that the secondary air valve 46 is closed in time.

The pressure difference dp represents the extent of the pressure change,which is produced between the points of time t1 and t2 and which is usedfor a secondary air injection. Without the idea for using the pressurechange for a secondary air injection the increased exhaust gas enthalpy,which results from the homogeneous split mode, would be ratherterminated by opening the waste gate valve 42.

1. A method of heating up a catalyst in an exhaust gas system of a charged combustion engine, the method comprising: adding air to the exhaust gas system, wherein the added air is taken from a suction system of the combustion engine in a direction of air current behind a compressor of an exhaust gas turbo charger that is arranged in the suction system; driving the combustion engine with a direct injection of a fuel into a plurality of combustion chambers, wherein the combustion engine is driven: after a start-up with an apportionment of a fuel amount injected before a beginning of a combustion, and wherein the fuel amount is apportioned into at least two partial injections per ignition per combustion chamber; and with a suboptimal ignition angle efficiency.
 2. A method according to claim 1, further comprising injecting the fuel into each of the plurality of combustion chambers such that a rich fuel/air mixture develops in each of the plurality of combustion chambers.
 3. A method according to claim 2, further comprising injecting the fuel into each of the plurality of combustion chambers such that an exhaust gas lambda value is greater than or equal to 1 after adding air.
 4. A method according to claim 1, further comprising operating the combustion engine in an almost completely throttled state.
 5. A method according claim 1, further comprising operating the combustion engine after a start-up with an increased idle engine speed.
 6. A method according to claim 1, further comprising detecting an engine speed of the combustion engine at a start-up; and activating the heating-up of the catalyst when the engine speed exceeds a start-up engine speed threshold value.
 7. A method according to claim 1, further comprising using a combustion engine that is compatible with a jet-formed gasoline direct injection.
 8. A method according to claim 7, further comprising operating the combustion engine during an idle with an ignition angle between 25 degrees and 40 degrees after a top dead center.
 9. A control unit, especially a control unit of a supercharged combustion engine, configured to implement steps of a method of heating up a catalyst in an exhaust gas system of a charged combustion engine, the method comprising: adding air to the exhaust gas system, wherein the added air is taken from a suction system of the combustion engine in a direction of air current behind a compressor of an exhaust gas turbo charger that is arranged in the suction system; driving the combustion engine with a direct injection of a fuel into a plurality of combustion chambers, wherein the combustion engine is driven: after a start-up with an apportionment of a fuel amount injected before a beginning of a combustion, and wherein the fuel amount is apportioned into at least two partial injections per ignition per combustion chamber; and with a suboptimal ignition angle efficiency.
 10. The control unit of claim 9, further configured to implement steps comprising at least one of: injecting the fuel into the combustion chamber such that a rich fuel/air mixture develops in the plurality of combustion chambers; injecting the fuel into the plurality of combustion chambers such that an exhaust gas lambda value is greater than or equal to 1 after adding air; operating the combustion engine in an almost completely throttled state; operating the combustion engine after a start-up with an increased idle engine speed; detecting an engine speed of the combustion engine at a start-up; and activating the heating-up of the catalyst when the engine speed exceeds a start-up engine speed threshold value; and operating the combustion engine during an idle with an ignition angle between 25 degrees and 40 degrees after a top dead center. 